1. Field of the Invention
This invention relates to a linear pulse motor, and more particularly to a linear pulse motor in which a movable part is driven by interaction of the magnetic flux of permanent magnets with magnetic flux generated by energization of the coils.
2. Description of the Prior Art
A linear pulse motor is used, for example, for linearly moving and accurately positioning a head in a printer or an optical read-out apparatus, or a movable part in a numerically controlled machine tool.
FIG. 1 shows schematically a linear pulse motor of the Prior Art. A stationary body 1 is long and plate-like, being made of magnetic material. Teeth 1a are formed at regular pitches in the longitudinal direction of the stationary body 1. A movable body 2 consists of two iron cores 2a and 2b and a permanent magnet 2c between them. The iron cores 2a and 2b include downwardly extending magnetic poles 2a1, 2a2, and 2b1, 2b2 as magnetic teeth respectively. Coils 2d and 2e are wound on the magnetic poles 2a1, 2a2, and 2b1, 2b2, respectively. The winding directions of the coil 2d are opposite on the magnetic poles 2a1 and 2a2. Similarly, the winding directions of the coil 2e are opposite on the magnetic poles 2b1 and 2b2.
The magnetic poles or magnetic teeth 2a1 and 2a2 of the one iron core 2a are shifted from one another in phase by a half of pitch (1/2P) with respect to the magnetic teeth 1a of the stationary body 1. Similiarly, the magnetic poles or magnetic teeth 2b1 and 2b2 of the other iron core 2b are shifted from one another in phase, by a half of pitch (1/2P) with respect to the magnetic teeth 1a, where the pitch P is equal to the pitch of the magnetic teeth 1a of the stationary body 1. Further, the iron cores 2a and 2b are so combined with the permanent magnet 2c that their magnetic teeth 2a1, 2a2, 2b1 and 2b2 are shifted by a quarter of the pitch (1/4P) in phase from one another with respect to the magnetic teeth 1a of the stationary body 1, in the order of the magnetic teeth 2a1, 2b1, 2a2 and 2b2.
As well known, the coils 2d and 2e are energized and changed over in the following predetermined order: a positive DC current +I is supplied to the coil 2d for a predetermined time; when the coil 2d is deenergized, the other coil 2e is energized to supply the positive DC current +I for the predetermined time; a negative DC current -I is supplied to the coil 2d for the predetermined time; and when the coil 2d is deenergized, the other coil 2e is energized to supply the negative DC current -I for the predetermined time.
A magnetic flux .PHI. from the permanent magnet 2c flows as shown by the dash lines in the iron cores 2a, 2b and the stationary body 1. For example, when the positive DC current I is supplied to the coil 2d, the magnetic flux induced by the coil 2d is added to the magnetic flux .PHI. from the permanent magnet 2c in the magnetic teeth 2a1, while the former is cancelled with the latter in the other magnetic teeth 2a2. As shown in FIG. 1, a magnetic pull force is generated between the magnetic teeth 2a1 and one of the adjacent magnetic teeth 1a of the stationary body 1. The movable body 2 steps by the quarter of pitch (1/4P). Although magnetic pull forces are generated between the magnetic teeth 2b1, 2b2 and the magnetic teeth 1a of the stationary body 1, by the magnetic flux .PHI. from the permanent magnet 2c, they are very small in comparison with the magnetic pull force between the magnetic teeth 2a1 and the adjacent one of the magnetic teeth 1a of the stationary body 1. This is true because a magnetic pull force is generally proportional to a square of magnetic flux density in the gap.
When the coils 2d and 2e are energized and changed over in the predetermined order, the movable body 2 actually moves forwards (leftwards) step by step in 1/4 pitch. It appears to move smoothly and continuously forwards.
However, in the above construction, the magnetic pull force does not act on the movable body 2 in symmetry with respect to the center line C-C in the moving direction, rather it acts on the left or right portion of the movable body 2 in accordance with the change-over of the energization of the coils 2d and 2e. Although not shown, the movable body 2 is provided with a pair of front wheels (bearings) and a pair of rear wheels (bearings) which roll on the side end portions of the stationary body 1. The strength of the magnetic pull force acting on the wheels changes. When it is high in the front wheels, it is low in the rear wheels. When it is high in the rear wheels, it is low in the front wheels. Thus, the movable body 2 vibrates at its rear and front end portions, and it makes much noise. The noise level is considerably high.
Further, when the temperature of the movable body 2 changes in accordance with the condition of the energization of the coil and the ambient temperature, the iron cores 2a and 2b, and the permanent magnet 2c expand or contract, and so the pitches of the magnetic teeth 2a1, 2a2, 2b1 and 2b2 change. The pitch relationship between the magnetic teeth of the movable body 2 and stationary body 1 also change. this change of pitch deteriorates the stop accuracy or positioning accuracy of the movable body 2. As the result, the character pitches become irregular, for example, in the printer, and the printing quality is deteriorated.